Apparatus and methods for converting cryogenic fluid into gas

ABSTRACT

An apparatus for vaporizing a cryogenic fluid comprises a heat transfer fluid closed circulation loop at least partially defined by an ambient air heat exchanger, a heater, and a vaporizer. The heat exchanger is configured to direct air downward across a plurality of heat exchanger tubes. A cryogenic fluid flows through the vaporizer. The plurality of tubes comprise a first tube having a first fin of a height, h 1 , and a second tube spaced adjacent the first tube, where the second tube has a second fin of a height, h 2 , wherein h 2  is different from h 1 .

REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent application Ser. No. 10/869,461 filed on Jun. 15, 2004, and is also a Continuation of U.S. patent application Ser. No. 11/405,854 filed on Apr. 18, 2006, each of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cryogenic fluids. In another aspect, the present invention relates to apparatus and methods for processing, transporting and/or storing cryogenic fluids. In even another aspect, the present invention relates to apparatus and methods for converting a cryogenic fluid into a gas. In even another aspect, the present invention relates to methods and apparatus for processing, transporting and/or storing liquified natural gas (“LNG”). In still another aspect, the present invention relates to apparatus and methods for modifying and/or retrofitting cryogenic vaporization systems. In yet another aspect, it further relates to apparatus and methods for the transfer of heat from one fluid to another fluid, and more particularly to an air-heated heat exchanger for heating low-temperature fluids.

2. Description of the Related Art

Most conveniently, natural gas is transported via pipeline from the location where it is produced to the location where it is consumed. However, given certain barriers of geography, economics, and/or politics, transportation by pipeline is not always possible, economic or permitted. Without an effective way to transport the natural gas to a location where there is a commercial demand, the gas may be burned as it is produced, which is wasteful.

Liquefaction of the natural gas facilitates storage and transportation of the natural gas (a mixture of hydrocarbons, typically 65 to 99 percent methane, with smaller amounts of ethane, propane and butane). When natural gas is chilled to below its boiling point (in the neighborhood of −260 F depending upon the composition) it becomes an odorless, colorless liquid having a volume which is less than one six hundredth ( 1/600) of its volume at ambient atmospheric surface temperature and pressure. Thus, it will be appreciated that a 150,000 cubic meter LNG tanker ship is capable of carrying the equivalent of 3.2 billion cubic feet of natural gas.

It is not uncommon for natural gas to be produced in remote locations, such as Algeria, Borneo, or Indonesia, and then liquefied and shipped overseas in this manner to Europe, Japan, or the United States. Typically, the natural gas is gathered through one or more pipelines to a land-based liquefaction facility. The LNG is then loaded onto a tanker equipped with cryogenic compartments (such a tanker may be referred to as an LNG carrier or (“LNGC”) by pumping it through a relatively short pipeline. After the LNGC reaches the destination port, the LNG is offloaded by cryogenic pump to a land-based regasification facility, where it may be stored in a liquid state or regasified. If regasified, the resulting natural gas then may be distributed through a pipeline system to various locations where it is consumed.

In many circumstances, hot water or steam is used to heat the liquefied gas for vaporization. Unfortunately, such hot water or steam often freezes so as to give rise to the hazard of clogging up the evaporator. Various improvements in this process have heretofore been made. The evaporators presently used are mainly of the open rack type, intermediate fluid type and submerged combustion type.

Open rack type evaporators use sea water as a heat source for the vaporization of liquefied natural gas. These evaporators use once-through seawater flow on the outside of a heat exchanger as the source of heat for the vaporization. They do not block up from freezing water, are easy to operate and maintain, but they are expensive to build. They are widely used in Japan. Their use in the USA and Europe is limited and economically difficult to justify for several reasons. First the present permitting environment does not allow returning the seawater to the sea at a very cold temperature because of environmental concerns for marine life. The present permitting environment allows only a small decrease in temperature before returning the seawater back to the sea, which would require a very large sea water quantity to be pumped through the system, if the terminal vaporization capacity was designed for a commercial size as economics would require. Also coastal waters like those of the southern USA are often not clean and contain a lot of suspended solids, which could require filtration. In addition the sea water intake structure would have to be located far away from the evaporators in most cases because of location restraints or to get to deep and clean sea water at the intake. With these restraints the use of open rack type vaporizers in the USA is environmentally and economically not feasible.

Instead of vaporizing liquefied natural gas by direct heating with water or steam, evaporators of the intermediate fluid type use propane, fluorinated hydrocarbons or like refrigerant having a low freezing point. The refrigerant is heated with hot water or steam first to utilize the evaporation and condensation of the refrigerant for the vaporization of liquefied natural gas. Evaporators of this type are less expensive to build than those of the open rack-type but require heating means, such as a burner, for the preparation of hot water or steam and are therefore costly to operate due to fuel consumption.

Evaporators of the submerged combustion type comprise a tube immersed in water which is heated with a combustion gas injected thereinto from a burner. Like the intermediate fluid type, the evaporators of the submerged combustion type involve a fuel cost and are expensive to operate.

The patent art is replete with a number of patents directed to processes and apparatus for the vaporization of cryogenic fluids, such as for example LNG. For example, U.S. Pat. No. 4,170,115, issued on Oct. 9, 1979 to Ooka et al., describes an apparatus for vaporizing liquefied natural gas using estuarine water. This system is arranged in a series of heat exchangers of the indirect heating, intermediate fluid type. A multitubular concurrent heat exchanger is also utilized in conjunction with a multitubular countercurrent heat exchanger. As a result, salt water is used for the vaporization process. U.S. Pat. No. 4,224,802, issued on Sep. 30, 1980 to the same inventor, describes a variation on this type and also uses estuarine water in a multitubular heat exchanger.

U.S. Pat. No. 4,331,129, issued on May 25, 1982 to Hong et al., teaches the utilization of solar energy for LNG vaporization. The solar energy is used for heating a second fluid, such as water. This second fluid is passed into heat exchange relationship with the liquefied natural gas. The water contains a anti-freeze additive so as to prevent freezing of the water during the vaporization process.

U.S. Pat. No. 4,399,660, issued on Aug. 23, 1983 to Vogler, Jr. et al., describes an atmospheric vaporizer suitable for vaporizing cryogenic liquids on a continuous basis. This device employs heat absorbed from the ambient air. At least three substantially vertical passes are piped together. Each pass includes a center tube with a plurality of fins substantially equally spaced around the tube.

U.S. Pat. No. 5,251,452, issued on Oct. 12, 1993 to L. Z. Widder, also discloses an ambient air vaporizer and heater for cryogenic liquids. This apparatus utilizes a plurality of vertically mounted and parallelly connected heat exchange tubes. Each tube has a plurality of external fins and a plurality of internal peripheral passageways symmetrically arranged in fluid communication with a central opening. A so lid bar extends within the central opening for a predetermined length of each tube to increase the rate of heat transfer between the cryogenic fluid in its vapor phase and the ambient air. The fluid is raised from its boiling point at the bottom of the tubes to a temperature at the top suitable for manufacturing and other operations.

U.S. Pat. No. 5,819,542, issued on Oct. 13, 1998 to Christiansen et al., teaches a heat exchange device having a first heat exchanger for evaporation of LNG and a second heat exchanger for superheating of gaseous natural gas. The heat exchangers are arranged for heating these fluids by means of a heating medium and having an outlet which is connected to a mixing device for mixing the heated fluids with the corresponding unheated fluids. The heat exchangers comprise a common housing in which they are provided with separate passages for the fluids. The mixing device, constitutes a unit together with the housing and has a single mixing chamber with one single fluid outlet. In separate passages, there are provided valves for the supply of LNG in the housing and in the mixing chamber.

U.S. Pat. No. 6,622,492, issued Sep. 23, 2003, to Eyermann, discloses apparatus and process for vaporizing liquefied natural gas including the extraction of heat from ambient air to heat circulating water. The heat exchange process includes a heat exchanger for the vaporization of liquefied natural gas, a circulating water system, and a water tower extracting heat from the ambient air to heat the circulating water. To make the process work throughout the year the process may be supplemented by a submerged fired heater connected to the water tower basin.

U.S. Pat. No. 6,644,041, issued Nov. 11, 2003 to Eyermann, discloses a process for vaporizing liquefied natural gas including passing water into a water tower so as to elevate a temperature of the water, pumping the elevated temperature water through a first heat exchanger, passing a circulating fluid through the first heat exchanger so as to transfer heat from the elevated temperature water into the circulating fluid, passing the liquefied natural gas into a second heat exchanger, pumping the heated circulating fluid from the first heat exchanger into the second heat exchanger so as to transfer heat from the circulating fluid to the liquefied natural gas, and discharging vaporized natural gas from the second heat exchanger.

One approach to increase a heat exchanger's overall heat transfer rate is to increase the heat transfer surface by attachment of radial or longitudinal fins to the external surface of a heat exchanger tube. The art is filled with patents directed to finned-tube heat exchangers, and methods of using and making such finned-tube heat exchangers.

U.S. Pat. No. 4,901,667, issued Feb. 20, 1990 to Demetri discloses a gas-to-liquid heat exchanger formed by winding circular finned tubing into a helical coil having bare tubing wrapped around the coil such that it nests between adjacent turns of the finned tubing. Fittings at the inlets and outlets of both coils distribute the liquid stream so that a portion flows through each coil. The fan tube coil acts as a cooled baffle which directs the hot gas stream flowing over the finned tubes so that it contacts a greater portion of the finned tube external surface area at high velocity and increases the heat transfer effectiveness.

U.S. Pat. No. 5,472,047, issued Dec. 5, 1995 to Welkey discloses a heat exchanger tube bundle design for a shell and tube exchanger that eliminates the need for tube supports or baffles within a heat exchanger tube bundle. The Welkey tube bundle configuration uses a combination of bare tubes and longitudinally finned tubes positioned such that the longitudinal fins act as spacing and supporting means within the tube bundle. The longitudinal fins provide spacing and support substantially along the entire length of the tubes within the bundle and thereby eliminate the need for internal spacing or supporting means.

U.S. Pat. No. 5,848,638, issued Dec. 15, 1998 to Kim discloses a finned tube heat exchanger described therein as having a simple structure and increased heat exchanging performance. The heat exchanger has a plurality of fin plates spaced at regular intervals and arranged in parallel with one another, and a plurality of heat exchanger tubes extending through the fin plates and including a refrigerant fluid therein. Each of the fin plates has a plurality of strips projected from the surface thereof, and the strips include first to fifth rows of strips arranged between openings, which are disposed adjacent to one another, in a parallel relationship. The first row of strips is located near a leading edge of the fin plates and formed of two louverlike strips in a form of a trapezoid having a long side located on the upper stream of the air flow. Each of the second to fourth rows of strips is formed of one bridgelike strip in a form of a rectangle. The fifth row of strips is formed of two louverlike strips in a form of a trapezoid having a short side located on the upper stream of the air flow

U.S. Pat. No. 6,659,170, issued Dec. 9, 2003 to Kale discloses a finned-coil heat exchanger having a housing with spaced walls defining an internal chamber with air flowing from an upstream end to a downstream end, spaced transfer tubes with heat conducting media flowing therein from the downstream chamber end to the upstream chamber end, a series of spaced fins in contact with the tubes to transfer heat to flowing air, and a fan unit to move air through the exchanger. An air inlet is defined at the upstream end of the housing or in the lower end of one of the walls so that air can enter the internal chamber. The tubes each extend tortuously back and forth on a plane parallel to the direction of air flow so that there is a counterflow effect across the various segments of each tube. The tubes have at least six segments extending transversely across air flow with the tubes and fins being sized and spaced to provide for better air flow through the heat exchanger housing.

U.S. Pat. No. 6,662,858, issued Dec. 16, 2003 to Wang discloses a counter flow heat exchanger with integrated fins and tubes comprising metal plates overlapping with each other. Each of the metal plates has multiple elongated ridges spacing apart from each other. Adjacent metal plates oppositely overlap with each other such that the ridges in pairs form horizontal tubes and multiple connecting tubes on the plates form vertical tubes. Fluid inside the heat exchanger flows counter to external air allowing heat exchange to be reached effectively.

U.S. Pat. No. 6,789,317, issued Sep. 14, 2004 to Sohal et al., discloses a system for and method of manufacturing a finned tube for a heat exchanger. A continuous fin strip is provided with at least one pair of vortex generators. A tube is rotated and linearly displaced while the continuous fin strip with vortex generators is spirally wrapped around the tube.

U.S. Pat. No. 6,928,833, issued Aug. 16, 2005 to Watanabe et al., discloses a heat exchanger finned tube for use in fabricating a heat exchanger useful as the evaporator for refrigerators or the like wherein a hydrocarbon refrigerant is used. Two tube insertion holes spaced apart from each other are formed in each of plate fins and two straight tube portions of a hairpin tube are inserted through the respective holes of each plate fin to arrange the plate fins in parallel into a plurality of fin groups spaced apart on the straight tube portions longitudinally thereof. The hairpin tube is enlarged with use of a fluid to fixedly fit the plate fins of each tin group around an enlarged tube portion of the hairpin tube and provide a finless part between each pair of adjacent fin groups on each of the straight tube portions. The heat exchanger fabricated using the finned tube exhibits the desired refrigeration performance with the leakage of refrigerant diminished.

In spite of the advancements of the prior art, there is still a need in the art for apparatus and methods for converting a liquified cryogenic fluid into a gas.

There is even another need in the art for apparatus and methods for converting a liquified natural gas into gaseous natural gas.

This and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.

SUMMARY OF THE INVENTION

The following presents a general summary of several aspects of the invention in order to provide a basic understanding of at least some facets of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key or critical elements of the invention or to limit the scope of the claims. The following summary merely presents some concepts of the invention in a general form as a prelude to the more detailed description that follows.

According to one aspect of the present invention, an apparatus for vaporizing a cryogenic fluid comprises a heat transfer fluid closed circulation loop at least partially defined by an ambient air heat exchanger, a heater, and a vaporizer. The heat exchanger is configured to direct air downward across a plurality of heat exchanger tubes. A cryogenic fluid flows through the vaporizer. The plurality of tubes comprise a first tube having a first fin of a height, h₁, and a second tube spaced adjacent the first tube, where the second tube has a second fin of a height, h₂, wherein h₂ is different from h₁.

According to another aspect of the present invention, a method of vaporizing a cryogenic fluid comprises transferring heat from ambient air to a heat transfer fluid by circulating the heat transfer fluid in a closed loop through a plurality of tubes of an air heat exchanger while directing ambient air downwardly across the plurality of tubes in the ambient air heat exchanger. A portion of the heat from the heat transfer fluid is used to vaporize at least a portion of the cryogenic fluid. The plurality of tubes comprise a first tube having a first fin of a height, h₁, and a second tube spaced adjacent the first tube. The second tube has a second fin of a height, h₂, where h₂ is different from h₁.

These and other embodiments of the present invention will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For detailed understanding of the illustrative embodiments, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein:

FIG. 1 is a process flow schematic showing a regasification system;

FIG. 2 is a process flow schematic showing another regasification system;

FIGS. 3 and 4 are schematics showing a retrofit of a typical ethylene glycol LNG vaporization system;

FIG. 5 is a schematic showing a retrofit of a water bath or submerged combustion system;

FIGS. 6 and 7 are schematics showing a retrofit of a typical cooling tower vaporization system;

FIG. 8 is a process flow schematic showing a vaporization process; and

FIG. 9 is a process flow schematic showing a vaporization process system;

FIG. 10 is a sketch of one embodiment of a tube arrangement for an air-heated exchanger according to the present invention;

FIG. 11 is a sketch of a staggered tube arrangement;

FIG. 12 is a sketch of a tube arrangement having eight rows of tubes;

FIG. 13 is a process flow schematic showing a retrofit of a typical ethylene glycol LNG vaporization system;

FIGS. 14 and 15 are schematics showing a retrofit of a typical cooling tower vaporization system; and

FIG. 16 is a process flow schematic showing a vaporization process system.

DETAILED DESCRIPTION

While some descriptions of the present invention may make reference to liquified natural gas (LNG), it should be understood that the present invention is not limited to utility with LNG, but rather has broad utility with cryogenic fluids in general, preferably cryogenic fluids formed from flammable gases.

The apparatus of the present invention will find utility for processing, storing, and/or transporting (i.e., including but not limited to, receiving, dispensing, distributing, moving) cryogenic fluids, a non-limiting example of which is liquified natural gas. More specifically, the present invention provides apparatus and methods for converting a cryogenic fluid into a gas, which apparatus and methods may be used not only be used in a stand alone manner, but which may also be utilized with and/or incorporated into apparatus and methods for processing, storing, and/or transporting cryogenic fluids.

According to the present invention, there is an unexpected advantage obtained using an air exchanger as a heater while passing a fluid through the tubes to pick up heat from the air, preferable on a continuous basis. While ambient air vaporizers have been used to heat LNG, they commonly require alternating between a number of units as they freeze up from water vapor in the air. The present invention employs an intermediate heat transfer fluid and selected temperatures, wherein frost does not inhibit the transfer surface during the vaporization of the LNG.

A first non-limiting embodiment of the apparatus and methods of the present invention is best described by reference to FIG. 1, a process flow schematic showing regasification system 100 having air exchange pre-heater 101, economizer 103, heater 105, water knockout 111, vaporizer 114, produced water pump 117, circulating fluid surge tank 119, and circulating fluid pump 121.

LNG is provided to vaporizer 114 via piping 21 at around −252 F, and exits vaporizer 114 via piping 22 as gaseous natural gas at about 40 F. A circulating heat transfer fluid is provided to vaporizer 114 via piping 31, and exits vaporizer 114 via piping 32 as a cooled heat transfer fluid.

Heat transfer fluids suitable for use in the present invention include hydrocarbons, non-limiting examples of which include propane and butane, ammonia, glycol-water mixtures, formate-water mixtures, methanol, propanol, and other suitable heat transfer fluids as may be useful under the operating conditions.

The heat transfer fluid is circulated in a closed system through air exchange pre-heater 101 where it is first heated after being cooled in vaporizer 114, then through economizer 103/heater 105 where it may be further heated if necessary, then through vaporizer 114 where it is utilized to provide heat of vaporization to the LNG, before returning to pre-heater 101. This heat transfer circulation system may be provided with one or more surge tanks 119 as necessary. Circulation of the heat transfer fluid is maintained by one or more circulation pumps 121. A nitrogen line 51 and pressure controller 55 maintain pressure of the heat transfer circulation system as desired.

In the practice of the present invention, heat is provided from ambient air to the heat transfer fluid across a heat transfer surface rather than by direct contact between the ambient air and heat transfer fluid. For example, the heat transfer fluid is passed through the tubes of a heat exchanger while the ambient air passes through the shell side.

Under certain conditions (see Examples 1, 2 and 3 below), ambient air will provide all of the heating necessary without the need for the economizer 103/heater 105 providing any heating duty.

When heater 105 is necessary it will be most efficiently run in conjunction with economizer 103, in which the exit effluent from heater 105 routed to economizer 103 to heat the LNG or other cryogenic fluid. The cooled effluent exits economizer 103 and flows to water knockout tank 111. Pump 117 eliminates produced water from the system.

A second non-limiting embodiment of the apparatus and methods of the present invention is best described by reference to FIG. 2, which is a process flow schematic showing regasification system 200 having tube-in-tube air exchanger 201, economizer 203, vaporizer 214, produced water knockout 211, produced water pump 217, warming medium accumulator 219 and warming medium pump 221.

In this embodiment, heat exchanger 201 is a tube-in-tube air exchanger (i.e., two tubes arranged in a concentric fashion), in which the cryogenic fluid passes through the inner most tube, pump 221 circulates the heat transfer fluid through the annular space between the two tubes, and ambient air passes over the surface of the outer tube. Accumulator 219 provides volume to the system to aid in heat transfer. For those times when the ambient air is too cool, extra heating may be provided by heater 214/economizer 203. Hot exit effluent from heater 214 routed to economizer 203 to heat the LNG or other cryogenic fluid. The cooled effluent exits economizer 203 and flows to water knockout tank 211. Pump 217 eliminates produced water from the system.

The methods and apparatus of the present invention also provide for retrofitting of pre-existing cryogenic regassification apparatus.

In its simplest aspect, regassification which involves closed loop circulation of a heat transfer fluid thru a heater and then into a vaporizer to heat and vaporize a cryogenic fluid, may be modified by placing an ambient air heat exchanger ahead of the heater to either pre-heat or fully heat the heat transfer fluid. Of course, there will not be direct contact of the heat transfer fluid with the ambient air, but rather indirect contact across a heat transfer service.

For example, referring now to FIG. 3, there is shown a retrofit of a typical ethylene glycol LNG vaporization system 300 having heater 302, LNG vaporizer 301, accumulator 303, and circulation pump 307. In a method of retrofitting/modifying the system to form a retrofitted/modified system, air pre-heater 315 is added just upstream of heater 302 to serve as a pre-heater and/or heater.

Referring now to FIG. 6, there is shown a retrofit of a typical cooling tower vaporization system 400, having cooling tower 401, pump 403, exchanger 404, tank 405, LNG vaporizer 406, pump 407 and submerged bath heater 408. In a method of retrofitting/modifying the system to form a retrofitted/modified system, air pre-heater 415 is added. However, instead of preheating the LNG, this heater 415 serves to heat the heat transfer fluid flowing through vaporizer 406.

More complex modification/retrofitting of such existing systems involve taking a side stream of the cryogenic fluid and routing it thru a vaporizer in which the vaporizer heat transfer fluid has been heated by ambient air in the manner of the present invention. Essentially, such a retrofit is the addition of the apparatus and method of the present invention to handle at least a portion of the vaporization.

For example, the typical ethylene glycol/water system shown in FIG. 3 and modified/retrofitted by the addition of air pre-heater 315, may instead be modified/retrofitted as shown in FIG. 4 by the addition of system 500 in which a heat transfer fluid is circulated in a closed circuit via pump 505 through air heater 502 where it is heated, through exchanger 501 where it heats LNG, through accumulator 503, and back to heater 502 to complete the circuit. Controller 509 regulates flow of LNG to the pipeline and/or back to the LNG Vaporizer.

As another example, the typical cooling tower vaporization system shown in FIG. 6 and modified/retrofitted by the addition of air exchanger 415, may instead be modified/retrofitted as shown in FIG. 7 by the addition of system 500 in which a heat transfer fluid is circulated in a closed circuit via pump 505 through air heater 502 where it is heated, through exchanger 501 where it heats LNG, through accumulator 503, and back to heater 502 to complete the circuit. Controller 509 regulates flow of LNG to the pipeline and/or back to the LNG Vaporizer.

FIG. 5 is a schematic showing the retrofit of a water bath or submerged combustion vaporizer by the addition of system 500 as described above.

Another non-limiting embodiment of the apparatus and methods of the present invention is best described by reference to FIG. 8, which is a process flow schematic showing vaporization process system 800 having air exchange pre-heater 801, accumulator 804, auxiliary heater 805, vaporizer 814, air exchange feeder line valve 816, heater feeder line valve 818 and temperature controller 825.

In this embodiment, temperature controller 825 monitors the temperature of the heat transfer fluid. If the temperature of the heat transfer fluid i s not sufficiently high, then controller 825 operates valves 816 and 818 to achieve a desired heat transfer fluid temperature, by utilizing pre-beater 801, auxiliary heater 805, or a combination thereof with the heating duty shared between heaters 801 and 805 in any suitable ratio. Controller 825 can be equipped with suitable algorithms in the form of either software and/or hardware to carry out this temperature control.

Another embodiment of the present invention is shown in FIG. 9, which is a process flow schematic showing vaporization process system 900 having air exchange pre-heater 901, auxiliary heater vaporizer 903, cold separator 904, auxiliary heater 905, second fluid pump 910, pre-heater vaporizer 914, pre-beater vaporizer LNG feed valve 916, auxiliary heater vaporizer LNG feed valve 918, second air exchange heater 920 and temperature controller 925.

This embodiment contains a pair of vaporizers in which vaporizer 903 receives heat transfer liquid from heater 905 and the other vaporizer 914 receives heat from heater 901. Temperature controller 925 monitors the temperature of gas 930 and operates valves 916 and 918 according to an algorithm to achieve the desired temperature of gas 930. The vaporization load is carried by the auxiliary heater vaporizer 903 and pre-heater vaporizer 914, or a combination thereof with the vaporization load shared between vaporizers 903 and 914 in any suitable ratio.

FIG. 10 illustrates one embodiment of a heat exchanger of the present invention, described above as an air exchange pre-heater or an air heater. As used herein air flows on one side of a heat transfer surface and a cryogenic fluid, or a secondary heat transfer fluid, flows on the other side the heat transfer surface. As shown in FIG. 10, air-heated heat exchanger 5 comprises tubes 160, 170, 180 arranged horizontally such that tube 160 is vertically spaced adjacent tube 170. Likewise, tube 170 is vertically spaced adjacent tube 180. Fluid 13 flows through the tubes from lower tube 180 sequentially through tube 170 and tube 160. Each of the tubes 160, 170, and 180 may have fins 15, 16, and 17 attached respectively thereto. Fins 15, 16, and 17 have height h₃, h₂, and h₁, respectively. Ambient air 120 is forced to flow downward across the tubes in essentially a counter flow exchanger arrangement by fan 110 driven by electric motor 90. While shown as a single fan, multiple fans may be used. Alternatively, any combination of fans and blowers of types known in the art may be used to force air across tubes 160, 170, and 180.

Fins 15, 16, and 17 may be of any type known in the art, such as, for example, spiral fins, and L-shaped fins. The fins act to increase the effective heat transfer surface area of each tube. The tubes and fins of the present invention may be constructed from any suitable material known in the art including steel, copper, aluminum, and alloys. The surface of the fins may be plain or they may be perforated, serrated, or comprise ripples, wrinkles, or bumps. These features improve the heat transfer from the surface of the fin to the air by increasing the fin surface area, increasing turbulence and reducing air bypass.

Also referring to FIG. 11, in operation, each of tubes 160, 170, and 180 represents a row of tubes. FIG. 11 depicts a staggered arrangement of tubes. By arranging tubes in a staggered, also called triangular, pattern, with transversely oriented rows of tubes staggered, the tubes can be closer together while still maintaining a sufficient open area percentage for airflow through the exchanger. For example, in a typical equilateral spacing of 2.5 inches (63.5 mm) between tubes having 1 inch (25.4 mm) diameter, the open area at any row of the coil (1 row % open) is about 60%. Also, the air passing through the coil is forced to go over and around each succeeding row of tubes. When a second staggered row is considered in the open area calculation, then the projected open area (2 row % open) nominally becomes about 20%. The triangular pattern significantly reduces bypass air without causing high pressure drops, and although tubes are partially “shadowed”, the increased air turbulence provides better air flow to the “shadowed” spots.

Alternatively, the tubes can be arranged in straight rows (not shown) and columns. Some advantages are obtained from the relative simplicity of such an arrangement. However, such an arrangement allows for a relatively high amount of bypass air. Another problem arises in that, except for the air side tube, each tube in a column is directly in the “shadow” of another tube, and does not receive an adequate flow of air. As a result, the most important portions of the fins, which are closest to the tubes, are in the “shadows” and do not receive adequate air flow, either.

Fin density is determined on an application dependent basis using techniques known in the art. Fin density may range from 4 fins/inch to 20 fins/inch. More commonly, the fin density is about 8-10 fins/inch. Fin height ranges from 0 to about ⅝inch. Tube diameters range form ½-4 inches. Tube spacing ranges from about 1¼-4 inches for 1 inch diameter tubes, with 1¼- 2¾ inches being more common, and from about 2¼-5 inches for 1.5 inch diameter tubes, with 2¼- 3¼ inches being more common. Tubes may be in the range of 5 to 60 feet in length. For long sections, multiple fans may be used, as described previously.

In one embodiment fluid 13 is a low-temperature secondary fluid having an operating temperature in the range of about −15° F. to about 30° F. As used herein, a secondary fluid is a fluid used in a closed-loop circulation system to act as an intermediate medium to transfer heat from the ambient air 120 to a third fluid (not shown), for example a liquefied natural gas (LNG), that is being heated and/or vaporized. Examples of such systems are described above. Secondary fluids for use as fluid 13 include, but are not limited to: (i) aqueous potassium formate solution, (ii) propane, (iii) refrigerant R22, (iv) ammonia; and (v) glycol/water solution. Alternatively, fluid 13 may be a primary fluid, such as, a cryogenic fluid, for example LNG, that is heated and/or vaporized as it flows through tubes 160, 170, and 180.

Ambient air 120, see FIG. 10, is forced to flow downward across the tubes in essentially a counter flow exchanger arrangement by fan 110 driven by electric motor 90. As one skilled in the art will appreciate, the temperature of air 120 will decrease as it passes downward through exchanger 5, while the temperature of fluid 13 will increase as it moves upward through exchanger 5. In one embodiment, lower tube 180 is a bare tube without extending fins. The rate of flow of air 120 through exchanger 5 is selected to allow condensed liquid 14 from air 120 to condense on the outside of tube 180. The condensed liquid 14 provides enhanced heat transfer from fluid 13, inside tube 180, to air 120. The relatively high latent heat of condensation of water in the condensed liquid 14 from air 120 is a substantially greater source of energy to heat fluid 13 than would be heating by straight convection. The selection of air flow based on the flow of coolant and the ambient temperature and humidity of the air to maintain a condensing liquid on lower tube 180 is within the capability of those skilled in the art. The selection of the tube material and fin height for each row in view of the ambient conditions provides a controlled condensation that allows operation of the exchanger without a significant frost buildup and therefore substantially eliminates the need for defrosting of the exchanger tubes.

Fin heights h₃, h₂, and h₁, are each selected to obtain a temperature profile through the exchanger to enhance the condensation of condensed liquid 14 on lower tube 180. While described above with respect to three rows of tubes, one skilled in the art will appreciate that any suitable numbers of rows of exchanger tubes may be stacked. For example, FIG. 12 shows an exchanger tube arrangement having eight rows of tubes arranged with several different fin heights. Four tubes 51 have fin 60 with height h₄. Two tubes 52 have fin 61 with height h₅, while the remaining two tubes 63 are bare tubes with no fins having condensed liquid 57 condensing thereon. Ambient air flow 55 is forced downward across the tubes by fan 64 while a fluid 56 travels upward sequentially through each successive tube. Shroud 66 may surround fan 64 to enhance air flow over the tubes. Such an exchanger tube arrangement may be used with either a secondary fluid loop or a primary fluid loop, as described above.

While described herein as a substantially horizontal assembly, it is contemplated that the present invention covers applications where the tubes are inclined from the horizontal up to about 70°.

It should be understood that any of the above systems may incorporate process controls/methods as are known to those of skill in the art. For example, by-passes around any of the heat exchanges may be utilized. It should also be understood that much of the engineering/process detail is not shown in the above illustrations but would be well within the knowledge and understanding of those of skill in the art.

Examples of such modifications include those shown on FIGS. 13-16. FIG. 13 shows a non-limiting example of a modified flow system similar to that of FIG. 4 but having block valve 320 inserted to control and/or isolate flow to vaporizer 301 and exchanger 501. FIG. 14 shows a non-limiting example of a modified flow system similar to that of FIG. 6 with heater 415′ shown in an alternative location. FIG. 15 shows a non-limiting example of a flow system similar to that of FIG. 7 having a portion of the inlet flow to submerged bath heater 408 branch off to exchanger 404. FIG. 16 shows a non-limiting example of a flow system similar to that of FIG. 9 having an additional outlet line 931 connecting to line 930. Other modifications are within the knowledge of those skilled in the art.

EXAMPLES

The following non-limiting examples are provided merely to illustrate a few embodiments of the present invention, and there examples are not meant to and do not limit the scope of the claims of the present invention. These inventions are theoretical calculated examples.

EXAMPLE 1

This example utilizes the apparatus and method as shown in FIG. 1 (11 at −10 F, 31 at 50 F, 32 at −10 F, and 119 at 16 psig). The cryogenic fluid is a typical LNG. The circulating fluid utilized is propane. The duty percentage for the air cooler 101, and the combined duty percentage for fired heater 105 and economizer 103 were calculated for ambient air temperatures of 35 F, 45 F, 65 F, 70 F and 85 F, with these percentages presented in the following TABLE 1. The propane circulation is about 1.7 lb propane/lb LNG, with the rate depending upon the temperature and pressure of the LNG and propane. The propane circulation range is estimated to be from about 1.0 to 2.5 lb propane/lb LNG. TABLE 1 Duty Percentage at Various Ambient Air Temperatures 85 F. 70 F. 65 F. 45 F. 35 F. Air Cooler 100 100 95 70 58 Fired Heat/Economizer 0 0 5 30 42 Total 100 100 100 100 100

EXAMPLE 2

This example also utilizes the apparatus and method as shown in FIG. 1 (11 @ −10 F, 31 @ 50 F, 32 @ −10 F, and 119 at 100 psig). The cryogenic fluid is again a typical LNG. The circulating fluid utilized is propane. The duty percentage for the air cooler 101, and the combined duty percentage for fired heater 105 and economizer 103 were calculated for ambient air temperatures of 35 F, 45 F, 65 F, 70 F and 85 F, with these percentages presented in the following TABLE 2. The propane circulation is about 7.6 lb propane/lb LNG, with the rate depending upon the temperature and pressure of the LNG and propane. The propane circulation range is estimated to be from about 5.0 to 10.0 lb propane/lb LNG. TABLE 2 Duty Percentage at Various Ambient Air Temperatures 85 F. 70 F. 65 F. 45 F. 35 F. Air Cooler 100 100 93 57 47 Fired Heat/Economizer 0 0 7 43 53 Total 100 100 100 100 100

EXAMPLE 3

This example again utilizes the apparatus and method as shown in FIG. 1 (11 at range of −10 F to 30 F, 30 at −10 F, 31 at 50 F, 32 at 30 F, and 119 at 16 psig). The cryogenic fluid is a typical LNG. Rather than using propane as the circulating fluid, WBF is utilized. As with Examples 1 and 2, the duty percentage for the air cooler 101, and the combined duty percentage for fired heater 105 and economizer 103 were calculated for ambient air temperatures of 35 F, 45 F, 65 F, 70 F and 85 F, with these percentages presented in the following TABLE 3. The WBF circulation is about 10-30 lb WBF/lb LNG, with the rate depending upon the temperature and pressure of the LNG and propane. TABLE 3 Duty Percentage at Various Ambient Air Temperatures 85 F. 70 F. 65 F. 45 F. 35 F. Air Cooler 100 100 93 60 51 Fired Heat/Economizer 0 0 7 40 49 Total 100 100 100 100 100

EXAMPLE 4

This example utilizes the apparatus and method as shown in FIG. 2. The cryogenic fluid is a typical LNG. The warming medium utilized is propane. The duty percentage for the tube-in-tube air exchange 201, and the combined duty percentage for fired heater 214 and economizer 203 were calculated for ambient air temperatures of 35 F, 45 F, 65 F, 70 F and 85 F, with these percentages presented in the following TABLE 4. The economizer is used with the Water Bath Heater only. TABLE 4 Duty Percentage at Various Ambient Air Temperatures 85 F. 70 F. 65 F. 45 F. 35 F. Air Cooler 100 100 93 57 47 Fired Heat/Economizer 0 0 5 43 53 Total 100 100 100 100 100

EXAMPLE 5

Potential savings utilizing present invention.

Basis: 1000 MMBtu/Hr; Air exchanger designed assuming 70 F; $5.00/MMBtu; 365 days of operation/yr. Month: January February March April May June July August September October November December T (F.): 51 54 61 67 75 81 82 83 79 70 61 55 AIR Htr % Duty: 77.5 81 90 94 100 100 100 100 100 100 90 80 Air Duty 775 810 900 940 1000 1000 1000 1000 1000 1000 900 800 (MMBtu/Hr):

Average Yearly Savings: 927.1×$5×24×365=$40.6 MM/Yr.

The above calculations are based on approximately 1500 MMSCFD being vaporized.

While the illustrative embodiments of the invention have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present invention, including all features which would be treated as equivalents thereof by those skilled in the art to which this invention pertains. 

1. An apparatus for vaporizing a cryogenic fluid, comprising: a heat transfer fluid closed circulation loop at least partially defined by an ambient air heat exchanger, a heater, and a vaporizer, wherein the heat exchanger is configured to direct air downward across a plurality of heat exchanger tubes; a cryogenic fluid flow path through the vaporizer; wherein, the plurality of tubes comprise: a first tube having a first fin of a height, h₁; and a second tube spaced adjacent the first tube, the second tube having a second fin of a height, h₂, wherein h₂ is different from h₁.
 2. The apparatus of claim 1, wherein the cryogenic fluid comprises liquefied natural gas (LNG).
 3. The apparatus of claim 2, wherein h₁ is less than h₂.
 4. The apparatus of claim 2, further comprising a third tube spaced adjacent the second tube, the third tube having a third fin of a height, h₃.
 5. The apparatus of claim 4, wherein h₂ is less than h₃.
 6. The apparatus of claim 2, wherein h₁ is substantially zero.
 7. The apparatus of claim 2, wherein the first tube comprises a plurality of vertically spaced horizontal rows of first tubes.
 8. The apparatus of claim 2, wherein the second tube comprises a plurality of vertically spaced horizontal rows of second tubes.
 9. The apparatus of claim 4, wherein the third tube comprises a plurality of vertically spaced horizontal rows of third tubes.
 10. The apparatus of claim 1, wherein the heat transfer fluid is chosen from the group consisting of: (i) an aqueous potassium formate solution, (ii) propane, (iii) refrigerant R-22, (iv) ammonia, (v) a glycol/water solution; and (vi) liquefied natural gas.
 11. The apparatus of claim 1, wherein the apparatus is operated under conditions such that at least a portion of the plurality of tubes has moisture condensed thereon.
 12. The apparatus of claim 1, wherein the apparatus operates under conditions that do not require defrosting.
 13. A method of vaporizing a cryogenic fluid, comprising: transferring heat from ambient air to a heat transfer fluid by circulating the heat transfer fluid in a closed loop through a plurality of tubes of an air heat exchanger by directing ambient air downwardly across the plurality of tubes in the ambient air heat exchanger; and transferring a portion of the heat from the heat transfer fluid to vaporize at least a portion of the cryogenic fluid; wherein the plurality of tubes comprise: a first tube having a first fin of a height, h₁; and a second tube spaced adjacent the first tube, the second tube having a second fin of a height, h₂, wherein h₂ is different from h₁.
 14. The method of claim 13, wherein the apparatus operates under conditions such that at least a portion of the plurality of tubes has moisture condensed thereon.
 15. The method of claim 13, wherein the apparatus operates under conditions that do not require defrosting.
 16. The method of claim 13, wherein h₁ is less than h₂.
 17. The method of claim 13, further comprising a third tube spaced adjacent the second tube, the third tube having a third fin of a height, h3.
 18. The method of claim 17, wherein h₂ is less than h₃.
 19. The method of claim 16, wherein h₁ is substantially zero.
 20. The method of claim 16, wherein the heat transfer fluid is chosen from the group consisting of: (i) an aqueous potassium formate solution, (ii) propane, (iii) refrigerant R-22, (iv) ammonia, (v) a glycol/water solution; and (vi) liquefied natural gas. 